1. Field of the Invention
The present invention relates to a planographic original plate. More specifically, the present invention relates to a planographic original plate which can be engraved directly by visible rays or laser beams such as infrared rays according to digital signals, and which can be water-developed, or can be mounted directly on a printing machine and used for printing without a developing process.
2. Description of the Related Art
A conventional manufacturing process of a printing plate by a PS (presensitized plate) includes a wet developing process to remove image-wise the photosensitive layer prepared on a substrate after exposure, and post-treatment processes of the developed printing plate such as rinsing with water and desensitization by means of rinsing with water containing a surfactant, or a solution containing gum arabic and a starch derivative.
In recent years, the plate making and printing industries have had to face the problem of environmental pollution due to the alkaline waste water from the developing process. At the same time, these industries have had been pursuing the rationalization of the plate making process. Accordingly, there is a demand for an original printing plate which does not require the above-mentioned complicated wet developing process by means of a developing solution and post-treatment process, and which can be used for printing directly after exposure without further processing.
As an example of an original plate which does not require a developing process after exposure, U.S. Pat. No. 5,258,263 discloses a planographic printing plate having a photosensitive hydrophilic layer whose curing or insolubilization is accelerated in the exposed region together with a photosensitive hydrophobic layer laminated on a substrate. However, because of the two-layer structure of the plate, this printing plate has a drawback that the adhesion between the upper layer and the lower layer is too weak to print a large number of printings.
On the other hand, U.S. Pat. Nos. 5,353,705 and 5,379,698 disclose a planographic original plate having a silicone layer and a laser-thermosensitive layer beneath the silicone layer, so as to provide a plate which does not require wet developing after the formation of an image. Although these plates dispense with wet developing, they have the drawback of requiring treatment by rubbing or by a special roller in order to ensure the complete removal of the silicone layer with laser abrasion, and therefore the process is complicated.
Japanese Patent Application Laid-Open (JP-A) Nos. 5-77,574, 4-125,189, U.S. Pat. No. 5,187,047, and JP-A No. 62-195,646 disclose the preparation of a planographic original plate which does not require a developing process by using as the printing material a film which is made by the sulfonation of a polyolefin, and changing the surface hydrophilicity of the film by thermal writing. The drawback associated with this technique is that, although the developing process is not necessary, the thermal writing generates a noxious gas because an image is formed by desulfonation of the surface of the planographic original plate.
U.S. Pat. Nos. 5,102,771 and 5,225,316 disclose a planographic original plate which is made up of a combination of a polymer having an acid-susceptible group in a side chain and a photo acid generating agent so as to provide a planographic original plate which dispenses with a developing process. The drawback associated with this technique is that the durability of the planographic original plate is poor and the sharpness of the printed image is inferior, because the acid generated is a carboxylic acid and the level of hydrophilicity is too low to keep the plate clean.
JP-A No. 4-121,748 disclose a photosensitive material which is made up of a combination of a polymer having a sulfonate group in a side chain and an acid generating agent together with a dye. This technique, however, involves a developing process by use of an alkaline developing solution, and therefore it does not propose any system which employs water-processing or dispenses with a developing process.